The emerging field of molecular electronics holds the promise for further miniaturization of computer memory and logic circuits down to nanometer size. At nanoscale dimensions, electronic circuit elements cannot be fabricated by current photolithographic methods. Moreover, many of the conventional methods of transporting, amplifying, and switching currents would not apply when features shrink to a few-nanometer size. Molecular electronics (moletronics) should be able to provide molecular-size replacements for various elements of semiconductor electronics.
One of the major electronic elements is the rectifying diode, which is used for a variety of purposes in analog and digital electronic circuits. Previous proposals for single-molecule rectifying diodes have been mainly discussed in terms of the donor-acceptor mechanism of Aviram and Ratner (AR), see e.g. A. Aviram et al, “Molecular Rectifiers”, Chemical Physics Letters, Vol. 29, pp. 277–283 (1974) and J. C. Ellenbogen et al, “Architectures for molecular electronic computers: 1. Logic structures and an adder designed from molecular electronic diodes”, Proceedings of the IEEE, Vol. 88, No. 3, pp. 386–426 (2000).
According to the AR mechanism, the two sides of the molecule are covalently attached to two side groups, one of which donates and another withdraws electrons from the central backbone of the molecule. As a result, the electron energy levels in the two parts of the molecule shift in opposite directions. The highest occupied molecular orbital (HOMO) becomes located in one part of the molecule while the lowest unoccupied molecular orbital (LUMO) becomes located in another part of the molecule. This is analogous to n- and p-doping of ordinary semiconductors. Since the levels shift under applied external bias, the HOMO and LUMO align at some voltage VF, thereby facilitating the resonant transport of electrons through the entire length of the molecule. At this voltage VF, the current sharply increases, indicating the opening of the diode in the forward direction. In the reverse direction, the HOMO and LUMO shift away from each other. Therefore, the resonant current picks up when other levels, different from the HOMO and LUMO, align. Since the energy difference between any other two levels is smaller that the HOMO-LUMO gap, the alignment in the reverse direction occurs at a higher voltage VR>VF. Thus, in the voltage interval VR>V>VF, the molecule transmits current in the forward direction which is much larger than the current in the reverse direction; that is, the molecule becomes a rectifying diode.
While the mechanism described above indeed provides current rectification, it has two serious drawbacks that make any practical applications of such diodes very difficult, if not impossible. The first drawback is the large typical values of the HOMO-LUMO gap, EHL>2 eV. In order to align the HOMO with the LUMO, an external bias of at least twice the gap is needed. That puts the forward voltage at VF>4 V. This is, in general, an operating voltage that is too high for a molecular diode. Even if the molecule does not burn out under such a voltage, the voltage drop would be so large that any series connection of such diodes, required for digital logic, would be impossible. The second drawback originates from the fact that, in practice, energy level shifts in the donor-acceptor mechanism are much smaller than EHL. As a result, the reverse voltage is only fractionally larger that the forward voltage, (VR−VF)/VF<<1. At the same time, for memory applications, ratios of at least VR/VF>5 are desired.
Thus, the conventional AR rectifying mechanism operates at large absolute values of VF and small ratios VR/VF, while moletronic applications require small absolute values of VF and large ratios VR/VF. What is needed is a molecular diode that would operate at as small VF as possible and at as large VR as possible.
A related molecular diode was discussed by Metzger et al (see, R. M. Metzger et al, Journal of the American Chemical Society, Vol. 119, pp. 10455–10466 (1997); and C. Krzeminski et al, Physical Review B, Vol. 64, pp. 085405(1–6) (2001)). In those experiments, an AR-like donor-acceptor molecule was attached to a long aliphatic chain (such as —C15H31) that facilitated the formation of a Langmiur-Blodgett (LB) film on the surface of water. In measuring current-voltage (I-V) characteristics of a molecular monolayer, asymmetry was observed. They noted that the electrical rectification arises from the asymmetric profile of the electrostatic potential across the system. They predicted the behavior contrary to the Aviram-Ratner mechanism: they have asserted that the onset of current at a negative threshold (−1.1V) corresponds to the resonance of the LUMO level with the Fermi level of the donor (D+) side electrode in their case. However, because of the long insulating chain, the measured currents were of order of 10−15 Ampere per molecule. At such small electric currents, the dominant transport mechanism is most likely to be temperature-assisted or impurity-assisted hopping. Such conductivities are too low for the diodes to be of practical importance.
Thus, what is needed is a molecular diode that would be more conductive and be based on the more effective resonant tunneling transport mechanism.